Method of making paper insulated cable



June 28, 1966 o. G. GARNER ETAL 3,258,374

METHOD oF MAKING PAPER INSULATED CABLE Original Filed June 9. 1961 Lu n u 3 a n D a m JNVENToRs 2 OSCAR e. GARNER 5 BY Lows MEYERHoFF o l 5o eo 1o ao so i n (g) Q TEMPERATURE F ATTURNEYS United States Patent O 3,258,374 METHOD F MAKING PAPER lNSULATED CABLE Oscar G. Garner, Westfield, and Louis Meyerhofl, Me-

tuchen, NJ., assignors to General Cable Corporation, New York, N.Y., a corporation of New `lersey Original application .lune 9, 1961, Ser. No. 116,037,1aow Patent No. 3,194,871, dated July 13, 1965. Divided and this application Mar. 31, 1965, Ser. No. 444,351 4 Claims. (Cl. 156-56) This application is a division of our .application Serial No. 116,087, iiled June 9, 1961, now patent No. 3,194,871.

This invention relates to paper insulated cables and, more particularly, to a method for the manufacture of paper insulated cables in which lthe initial moisture contentof the applied paper tape is controlled to produce an improved ca'ble.

In paper insulated cables the insulation comprises a plurality of paper tapes lapplied in concentric helical wrappings over the conductor or cable core. Adjacent turns in each Wrapping are laid so that the edges are arranged in nearly abutting relationship. Overlying 'helical wrappings are staggered relative to each other to overlap the abutting edges. The paper tape contains varying amounts of moisture related primarily to the ambient relative humidity at the installation location. After the tapes are applied to the thickness required for the application intended, the built-up wall of insulation is dried in a vacuum oven.

In the vacuum oven, the combined action of heat and vacuum remove substantially all of the moisture from the paper insulation. Subsequently, insulating oils or compounds are introduced into the oven to saturate or impregnate the insulation.

The dimensions of the paper tape vary with the moisture content thereof. That is, with increased moisture content the paper tapes will swell, increasing the dimensions of the tape by an amount dependent upon the moisture content thereof, which in turn depends essentially upon the relative humidity of the ambient atmosphere.

If the initial moisture content of the tapes is not con trolled during application thereof, a varying and uncontrolled amount of shrinkage of the paper within the insulation wall, and concomitant variation in the compact' ness of the insulation, will result during drying in a vacuum oven.

Since the electrical strength of the insulation increases With the overall density of the insulation, the variation of shrinkage during drying leads to a similar variation in the electrical strength of the insulation. References herein to the density of the insulation, or a layer thereof, mean that proportion of the total volume occupied by the insulation, or the layer, which is made up of paper tape, and do not refer to the density of a single paper tape unless so specified.

The art has, in the past, stressed the value of uniformity of compactness to ensure product uniformity. For this purpose, some cable manufactures have controlled the relative humidity conditions in the rooms in which the paper taping heads are located so that the relative humidity therein ranges from about 40 to `60%. More recently, it has 'been proposed that the relative humidity at the paper taping head be controllably maintained at a lower relative humidity to further decrease the initial moisture content of the applied paper tapes and, thus, to increase the insulation. compactness.

The decrease -in the initial moisture content of the applied paper tape -by control of the relative humidity will, of course, be effective to increase the density of the insulation. However, concomitant with the Iapplication of such drier tapes is a disproportionate increase in the Patented JuneV 28, i966 ice . sulation core along a zigzag path through the spaces between adjacent turns and layers of the tape. lBy starting with initially drier tape, the shrinkage is reduced, producing smaller spaces 4between^`the tapes. The smaller spaces introduce higher ow resistance to the water -vapor and, thus, entails longer drying time. With a cable having a one-inch thick wall of insulation, the difference in drying time may be several days.

Further, uniformity of density in a radial direction is not essential since the electric stress varies as an inverse function of the distance from the core.

It is, therefore, one object of this invention to provide a method of manufacturing an improved cable construction having -high density insulation applied thereto without disproportionate increase in the required drying time.

It is another object of this invention to provide a method for the manufacture of an improved cable construction in which the insulation density is varied in accordance with the electrical stress by using papers of varying initial moisture content.

It is a'further object of this invention to provide an improved method for the manufacture of paper insulated cable in which the density of the insulation and uniformity of compactness is maintained without adversely increasing the drying time.

It is a still further object of this invention to provide an improved method of fabricating such cable in which the layer of paper insulation adjacent the cable core is applied under controlled low moisture content and the superimposed layers are applied with controlled, but higher, moisture content. As used herein: the term wrapping designates a single helically wrapped paper insulating tape; the term layer designates a plurality of overlying wrappings composed of paper tapes all having similar initial moisture content; and the term wall designates the entire body of the paper insulation comprising two or more of such layers, the densities of the insulation in the 4several layers differing from each other, after evacuation, by reason of the fact that the initi-al moisture content of the paper in one layer was different from that in the other layer or layers.

In accordance with these objects, we have provided, in accordance with a preferred embodiment of this invention, a paper insulated cable in which the portion or layer of the paper tapes applied directly on the cable core is applied with initial low moisture content and in which overlying layers of tape are applied with increased moisture content. In such cable, the shrinkage during drying of the paper tapes with high initial moisture content causes relatively wide spaces between tapes to improve the speed of moisture removal during treatment of the cable in a vacuum oven. The insulation immediately adjacent the cable core is, however, maintained at a higher density. The higher density adjacent the cable core provides the electrical strength necessary since this higher densi-ty is at the place of highest electrical stress in the cable insulation.

In accordance with the method of this invention, the cable is manufactured by assembling the cable core and passing it through a paper taping machine, comprising a plurality of taping heads, maintained in an atmosphere of controlled low humidity. The paper taping heads apply an inner layer of helically wrapped tapes of low moisture content for slightly less than half the thickness of the Wall required for the application intended. The

cable is then passed through a similar paper taping machine for application of the remainder of the required plurality of tapes which are applied as an outer layer of helical wrappings at a higher moisture content.

The cable is then placed in the usual vacuum oven for -tionate increase in drying time.

The invention may be more easily understood by reference to the following description taken in conjunction with the accompanying drawings, of which:

FIGURE 1 is a cross section of a paper insulated cable as initially fabricated in accordance with this invention;

FIGURE 2 is a cross sectional view of the cable shown in FIGURE 1 after drying of the paper insulation theref; FIGURES is a schematic illustration of a cable assembly operation illustrating the method of fabrication of the cable shown in FIGURES 1 and 2; land FIGURE 4 is a plot of moisture content versus tem'- perature for various relative humidities in which the moisture, as a percent of dry paper weight, is plotted along the scale of ordinates and temperature in degrees Fahrenheit is plotted along the scale of abscissa.

In FIGURES 1 and 2 there is shown a cable comprising a cable core upon which is applied a plurality of overlying, concentric, helical wrappings of paper tape 12 having a controllably low moisture content. On top of these tapes are applied a plurality of paper tape wrappings 14 having a controllably higher moisture content.

When subject to drying in a vacuum oven, the outer layer of tapes 14 will shrink more than the inner layer of tapes 12 to provide a relatively wide path between tape layers and, to lesser extent, between adjacent tapes. The spaces opened by such shrinkage will provide a parambulating path indicated by dotted line 16 through the paper wrappings 14 of the outer layer to the inner drier tapes 12. The inner tapes 12 applied with a lower ini-tial moisture content will, of course, not shrink to the same extreme and will provide a more compact and uniform layer of insulation. However, the paths thereto through the layer of outer tapes will increase the drying effect of the oven and prevent the great increase in drying time that will result if the entire insulating wall is formed of the drier tapes.

For understanding of the invention, it shall be necessary to specifically study the dimensional changes of paper undervarying degrees of moisture content and the mechanism of drying the paper in an applied wall of paper insulation.

It is known that the dimensions of the paper tape are affected by its moisture content. For example, conventional insulating tape having a moisture content of 7% (taken as a percentage of dry weight of paper) is about 1% longer, 2% wider and 4% thicker than fully dried paper.

The moisture' content of the paper will vary with the relative humidity and the temperature of the atmosphere as is shown in the graph of FIGURE 4, assuming equilibrium conditions have been reached. It will be noted that the temperature effect is much smaller than that of the relative humidity. It will further be noted that changes in the moisture content ofthe paper tape are not directly proportional to changes in atmospheric relative humidity. For example, change of the relative humidity from 60 to 30% decreases the moisture content from 7.8% to 4.5% at 75 temperature. Further, although control of the moisture content of the applied tape can, to some extent, be afforded by control of the relative humidity of the room in which the paper tapes are applied, it is still necessary to dry the tape in a vacuum oven to obtain the necessary dryness for paper-insulated oil-impregnated cable.

In the vacuum oven the temperature is raised to approximately C. The pressure difference between the vapor pressure of the moisture within the insulation and the vacuum maintained in the oven forces the moisture out into the oven from which it is removed by a vacuum pump. Initially, the vapor pressure is high and drying proceeds relatively rapidly. As the paper becomes drier, the vapor pressure becomes correspondingly lower and the rate of drying decreases. Thus, while it is relatively easy to remove the large bulk of the moisture, removal of the last few tenths of a percent is difficult and requires extended drying treatment.

The removal of the moisture from the interior of the plurality of applied paper tapes proceeds primarily through the zigzag paths provided by the butt spaces of adjacent tape turns in each wrapping and the spaces between overlying concentric tape wrappings. Therefore, the larger :the spaces between the tapes, the loweris the flow resistance, and therefore the faster is the rate of drying. Conversely, smaller spaces inhibit the flow of the vapor and increase .the drying time.

The increase in drying time is a drawback to the use of initially dry tapes in the fabrication of paper insulated cable. Although there is less moisture to remove from applied dry tape, the reduced shrinkage of such tape greatly increases the time necessary for drying the cable within the vacuum ovens. For example, if initially drier tapes (such as those applied under a controlled relative humidity of 10%) are used for the fabrication of a cable having a one-inch thick wall of paper insulation (e.g. for 345 kv. cable), the drying time will increase by as much as several days. This increase in drying time renders such cable uneconomical in conventional practice, despite the apparent advantages of higher insulation density and an increased uniformity of insulation.

It will be noted, however, that a concentrically insulated cable is essentially a-cylindrical condenser and the electrical stress throughout the insulation is inversely proportional to the diameter. For example, if the conductor diameter `is 1.7 and the outer diameter of theinsulation is 3.7", the electrical stress in the insulation at the conductor is 2.18 times the stress at the outermost part of the insulation and is 1.37 times the stress 'at the midpoint of the insulation.

Therefore, the specific electrical strength of the insulation must be higher adjacent the cable core than is necessary at the outer diameter of the insulation wall.

The increased electrical strength density of the insulation may be provided by increased density of the applied paper tape insulation. The increased density of paper tape insulation adjacent the core can be attained without substantially increasing the drying time of the entire paper insulation by the use of relatively dry'paper in the inner portion of the insulation and of less dry paper in the outer portion as illustrated in FIGURES 1 and 2. The use of higher density paper insulation is unnecessary over the entire wall.

During drying of such cable, the tapes at the outer layer will shrink providing increased path for moisture flow from the inner layer of drier paper tapes. Thus, there is afforded an inner layer of high density paper tape insulation with an overlying layer of paper tape insulation having a lower electrical strength, but positioned at a point of lower electrical stress.

The method of fabricating paper insulated cable in accordance with this invention is best seen by reference to FIGURE 3.

In FIGURE 3 there is shown the cable core 10 which is passed through a paper taping machine having a plurality of paper taping heads 20. Although only one paper taping head is illustrated for simplicity, a conventional taping machine may have twelve taping heads, each of which is provided with ten pad holders 22. Thus a total of 120 tapes 24 are applied to the core as it passes through the paper taping machine.

The paper taping'machine is maintained in an enclosure defined by Walls 26. The relative humidity within the enclosure is maintained low as, for example, at relative humidity. The paper tapes are allowed to reach equilibrium condition with the relative humidity of the room. While this might be done by storing the tape in the controlled room, it is usual to dry the tapes to the approximate moisture content desired by passing the tape over heated rolls. The mechanical drying of the tapes to approximately the equilibrium condition is much faster and usually found advisable.

As can be seen from FIGURE 4, at 10% relative humidity, the tapes will reach equilibrium at about 3% relative moisture. The tapes of initially low moisture content are applied until the insulation thickness is somewhat less than one-half of the desired insulation wall thickness. For the example taken, 120 tape wraps are applied at this stage.

The cable is then fed through a second paper taping machine having, for example, 14` paper taping heads 28 each of which has ten tape pads 30. Thus 140 tapes 32 are applied by the machine. The relative humidity of the enclosure 34 may be maintained relatively higher, for example, 30% relative humidity, so that the outer tapes are applied at a relatively higher initial moisture content of 4 to 4.5%, determinable from FIGURE 4.

Thus the entire cable insulation is built up from a plurality of relatively dry tape Wrappings adjacent the core over which is applied a plurality of relatively moist tape wrappings. On the drying of the cable insulation, the difference in the shrinkage of the tapes with different initial moisture content will result in a relatively high density insulation adjacent the core (i.e. at the position of highest electrical stress) and a relatively lower density insulation over the high density insulation.

This invention may be variously modied and embodied within the scope of the subjoined claims.

What is claimed is:

1. The `rnethod of fabricating paper insulated cable which comprises controlling the moisture content of the paper tape at a low moisture content by drying the tape to the desired low moisture content, applying the low moisture content tape in a rst plurality of layers adjacent the cable core in an atmosphere of such relative humidity that the moisture content of the dried tape remains in equilibrium, and applying a second plurality of layers of tape of similarly controlled, but higher, moisture content thereover.

2. The method in accordance with claim 1 in which the moisture content of the dryer tape of the first plurality of layers is maintained at below 3% moisture with respect to dry paper weight.

3. The method in accordance with claim 1 in which the moisture content of the relatively moister tape of the second plurality of layers is maintained above 4.0% moisture with respect to dry paper weight.

4. The method in accordance with claim 1 in which 'the idrier tape is applied in a plurality of layers comprising less than one half of the thickness of the desired wall.

References Cited by the Examiner UNITED STATES PATENTS 1,771,946 7/ 1930 Zapf. 2,190,017 2/ 1940 Del Mar.

FOREIGN PATENTS 149,006 2/ 1922 Great Britain. 151,667 10/ 1920 Great Britain. 364,710 1/ 1932 Great Britain. 832,397 4/ 1960 Great Britain.

EARL M. BERGERT, Primary Examiner.

J. MATHEWS, Assistant Examiner. 

1. THE METHOD OF FABRICATING PAPER INSULATED CABLE WHICH COMPRISES CONTROLLING THE MOISTURE CONTENT OF THE PAPER TAPE AT A LOW MOISTURE CONTENT BY DRYING THE TAPE TO THE DESIRED LOW MOISTURE CONTENT, APPLYING THE LOW MOISTURE CONTENT TAPE IN A FIRST PLURALITY OF LAYERS ADJACENT THE CABLE CORE IN AN ATMOSPHERE OF SUCH RELATIVE HUMIDITY THAT THE MOISTURE CONTENT OF THE DRIED TAPE REMAINS IN 